The invention refers to a rocket nozzle having axi- ally double bell shape, i.e. of so-called "Dual Bell" type, and having an outwardly directed change of curvature of the contour line or generatrix at the inflection point between the two bell shapes.

The Dual Bell shape of rocket nozzles is known from the early 60's for providing an altitude compensation. In sea level operation mode such a Dual Bell nozzle the in- flection point will force the flow to separate from the nozzle wall at the desired location, thus increasing sea level thrust. In altitude operation mode the plume gradu¬ ally expands until it finally attaches to the nozzle wall downstream of the inflection point. In reality, however, the Dual Bell nozzle concept has several inherent ineffi¬ ciencies which reduce its performance from the theoretical op imum.

On the other hand, the function of the rocket nozzle is to expand and accelerate the gas to high velocity and thereby give thrust efficiency and payload capacity. The thrust efficiency is especially important to upper rocket stages. High thrust performance means high wall tempera¬ tures and as a consequence leads to exotic and expensive technologies. The temperature of the walls of a rocket noz- zle is dependent on the pressure at the wall and the speed of the flow at the wall.

For controlling the wall temperature of a rocket noz¬ zle, particularly wall portions which are not actively cooled by convection cooling, several techniques have been suggested. First of all, the materials used are to have strength at very high temperatures which of course is ex-

pensive. The nozzle walls also may be covered by coatings that insulate and allow high surface temperatures. This is also expensive. Finally, a cooling film might be used in combination with a continuous nozzle contour. In the case of using metallic materials, such materi¬ als have high cost and a nozzle structure must be built with many joints due to the material availability. The large number of joints, however, lower the reliability. Al¬ ternatively, a ceramic matrix composite material may be used. In this case the cost is very high and the reliabil¬ ity might be questioned due to little experience for appli¬ cation in rocket nozzles.

Thus, coatings add cost and the potential to lower the steady state temperature is limited. A coating also means reduced reliability due to increased complexity. As to the case of film cooling, there is normally no gas to produce film available for closed cycle engines. Tapping of gas for film cooling purpose would mean serious performance loss. It has now turned out that a simple and inexpensive way to obtain a control of the temperature of the nozzle walls might be obtained based on the Dual Bell shape but adapted as suggested according to the present invention. The invention thus is substantially distinguished in that for obtaining an improved cooling action on the nozzle wall the change of curvature amounts to between 2° and 7°, said inflection point (I) being located between a location at the area ratio ε = 10 and a location at 0,85 x ε _max of the nozzle. By introduction of a discontinuity in the meridional plane for a nozzle contour the wall temperature will be

lowered faster than what would be the case for the normal continuous contour. The temperature of the nozzle wall from the point of the discontinuity is made close to constant . The temperature that decides the nozzle material therefore is lowered. As a side effect, by introduction of a discon¬ tinuity the behaviour of a cooling film could be con¬ trolled. At the inflection point the film close to the noz¬ zle wall will be subjected to a sudden acceleration just downstream of said discontinuity which will stabilise the film and prevent mixing. The efficiency of the film is then maintained.

The invention will be further described below by way of example with reference to the accompanying drawings in which Fig. 1 is a longitudinal section of a nozzle shape according to the present invention, the left half of which illustrates a location of the inflection point I at a loca¬ tion ε = 10 while the right half illustrates a location of the inflection point I at ε = 85 percent of ε _max , and Fig. 2 shows the curve of the relation between wall temperature and axial length of the nozzle according to the invention.

In Fig. 1 it is thus illustrated a rocket nozzle 1 of so-called "Dual Bell" type, i.e. having an axially double bell shape. Similar to well-known altitude compensating nozzle structures there is an inflection point I on the contour line or generatrix where there is a sudden change in the curvature of said contour line, in other words where the upper bell shape changes to the further bell shape next thereto. Unlike the known Dual Bell structures where the change of curvature amounts to at least 9° in order to pro- vide for a sudden direction change for obtaining the de-

sired separation of the flow along the nozzle wall at said point, the present invention suggests that the change of curvature amounts to only between 2 and 7°, the inflection point I being located between a location at the area ratio ε = 10 and the location at 0,85 x ε _max of the nozzle, ε is the area ratio which amounts to ε = 1 at the throat of the nozzle.

According to the invention the inflection point might be located at any suitable location between the two stipu- lated limits stated above.

The sudden acceleration of the film flow along the wall caused by the change of curvature of the wall contour line provides for an improved cooling effect starting imme¬ diately downstream said inflection point and maintaining said effect to such an extent that the rest of the wall downstream said inflection point will be kept almost con¬ stant . Said reduced wall temperature thus allows the use of a wall material not at all as temperature resistant as re¬ quested in prior art and consequently a cheaper structure.